In the United States, the annual incidence of myocarditis is estimated
at 1 to 10 per 100 000 population. As many as 1% to 5% of patients with
acute viral infections involve the myocardium. Although many viruses have
been reported to cause myopericarditis, it has been a rare or unrecognized
event after vaccination with the currently used strain of vaccinia virus (New
York City Board of Health).
To describe a series of probable cases of myopericarditis following
smallpox vaccination among US military service members reported since the
reintroduction of vaccinia vaccine.
Design, Setting, Participants
Surveillance case definitions are presented. The cases were identified
either through sentinel reporting to US military headquarters surveillance
using the Defense Medical Surveillance System or reports to the Vaccine Adverse
Event Reporting System using International Classification
of Diseases, Ninth Revision. The cases occurred among individuals vaccinated
from mid-December 2002 to March 14, 2003.
Main Outcome Measure
Elevated serum levels of creatine kinase (MB isoenzyme), troponin I,
and troponin T, usually in the presence of ST-segment elevation on electrocardiogram
and wall motion abnormalities on echocardiogram.
Among 230 734 primary vaccinees, 18 cases of probable myopericarditis
after smallpox vaccination were reported (an incidence of 7.8 per 100 000
over 30 days). No cases of myopericarditis following smallpox vaccination
were reported among 95 622 vaccinees who were previously vaccinated.
All cases were white men aged 21 years to 33 years (mean age, 26.5 years),
who presented with acute myopericarditis 7 to 19 days following vaccination.
A causal relationship is supported by the close temporal clustering (7-19
days; mean, 10.5 days following vaccination), wide geographic and temporal
distribution, occurrence in only primary vaccinees, and lack of evidence for
alternative etiologies or other diseases associated with myopericarditis.
Additional supporting evidence is the observation that the observed rate of
myopericarditis among primary vaccinees is 3.6-fold (95% confidence interval,
3.33-4.11) higher than the expected rate among personnel who were not vaccinated.
The background incidence of myopericarditis did not show statistical significance
when stratified by age (20-34 years: 2.18 expected cases per 100 000;
95% confidence interval [CI], 1.90-2.34), race (whites: 1.82 per 100 000;
95% CI, 1.50-2.01), and sex (males: 2.28 per 100 000; 95% CI, 2.04-2.54).
Among US military personnel vaccinated against smallpox, myopericarditis
occurred at a rate of 1 per 12 819 primary vaccinees. Myopericarditis
should be considered an expected adverse event associated with smallpox vaccination.
Clinicians should consider myopericarditis in the differential diagnosis of
patients presenting with chest pain 4 to 30 days following smallpox vaccination
and be aware of the implications as well as the need to report this potential